In WWI, concrete pill boxes were rare. Most machine gun emplacements were protected by sandbags and makeshift defenses. The defenders would have to be out in the open. When artillery rain down on them, they would stay in their bunkers which had no ports for them to fire out of. They would wait until the shelling stopped, get of their bunkers, and then man the machine guns.
The attackers, meanwhile, if they got their timing right... would be on top of the trenches when this happens. Giving the attackers a chance to engage the defenders before the machine guns open up.
The creeping barrage was "creeping" in some sense. This is what I don't get. Surely, it would've been enough to aim it at the opposing trench, and not have it "creep up" at all.
I feel like every explanation I'm getting now is deja vu.
This link has an image of the artillery plan for the Passchendaele campaign. The barrages were timed at 3 minute intervals. Advancing a hundred yards at time.
How?